And the shack? It's actually a million-dollar observatory packed with intricate instruments that probe the Arctic air. In fact, it's one of the world's most sophisticated stations for tracking changes in the mix of gases that can alter climate and destroy the Earth's protective ozone layer.

Recently, some of those data have been encouraging. While problems persist, a new survey of air samples from the Point Barrow Observatory and other sampling sites indicates that global agreements to curb the manufacture and use of chemicals that attack the ozone layer are working.

"You can see that what we're doing is making a difference," says Mr. Endres, station chief for the Barrow facility, referring to the agreements and the results.

Barrow's station is one of four run by the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (CMDL) in Boulder, Colo. The others are in Hawaii, American Samoa, and at the South Pole.

New findings

The recent review, led by CMDL chemist Stephen Montzka, shows that overall, ozone-depleting compounds in the lower atmosphere fell by about 3 percent between 1994, when they were at their peak, and 1997, the latest figures used for the study.

These compounds eventually work their way into the stratosphere, where they break down the ozone that protects the Earth's surface from excessive amounts of the sun's ultraviolet radiation. Such reactions have led to the ozone hole that appears over Antarctica each spring and to a more recent thinning of the ozone layer over parts of the Arctic, researchers say.

"One of the striking things from this study is how rapidly we eliminated methyl chloroform," says Dr. Montzka, talking about a chlorine-based compound that was once a widely used cleaning solvent.

Based on the provisions of the 1987 Montreal Protocol and subsequent amendments, the ozone layer is expected to recover to pre-1980 levels by 2050, with the first evidence of recovery likely to show up in about 10 years.

But several factors could delay the recovery, researchers say. Within CMDL data, which were published recently in the journal Nature, lie a pair of "red flags," says Michael Prather, an atmospheric scientist at the University of California at Irvine.

One is the increasing level of a chlorine-based compound known as chlorofluorocarbon 12. Used in refrigerators and air conditioners, CFC-12 is no longer produced in developed countries, although large amounts of the gas lie in junked appliances, which leak as they age.

But Dr. Prather adds that CFC-12 is still being produced in developing nations, such as China, Mexico, and India, which were given an extra 10 years to begin phasing out production. Also, emissions of one chemical thought to be a byproduct of making CFCs are occurring at rates higher than expected.

"CFC-12 is the sleeping giant," Prather says, noting that concentrations continue to rise and that the gas has a lifetime in the atmosphere of 100 to 110 years. "If we cannot control that in the long run, we've lost everything."

Another red flag is the continued use of fire-fighting compounds based on the chemical bromine, which is 30 to 50 times more effective than chlorine at destroying ozone.

Despite efforts to find alternatives and to recycle the bromide-based compound halon-1211, emissions have not eased, even though production was supposed to have ceased in developed countries in 1994. And like CFC-12, halon-1211 lies in "banks" of equipment whose use or leakage could sustain a relatively high level of emissions for as many as 12 more years.

"The excess is suggesting that someone is ramping up production," Prather says.

Moreover, climate and atmospheric conditions may have the potential to slow the ozone layer's recovery - particularly over the Arctic. Researchers are concerned about ozone holes in the Arctic because any hole, or fragments of it that drift south when a hole breaks up, may affect highly populated areas.

In March, for example, a team of German researchers suggested that some nitrogen - which hinders ozone-destroying reactions - may be permanently lost from the atmosphere because of global warming.

Their simulation is based on the theory that a warmer climate will accelerate a jet-stream-like river of air encircling the Arctic, effectively blocking warm air flowing north from the equator. The stratosphere will cool and clouds will form, meaning that nitrogen compounds will become part of the cloud droplets and fall out as precipitation.

The researchers, led by A.E. Waibel of the Max Planck Institute for Chemistry in Mainz, Germany, already note that because of a series of colder-than-usual Arctic winters, the region is on the verge of denitrification now.

Uncertain simulations

David Hoffmann, director of the CMDL in Boulder, warns that these simulations have large uncertainties and can give a wide range of results. "The first attempts to do these kinds of assessments show that everything goes the wrong way, prolonging the recovery," he says. "Modeling these is still in its infancy."

"We've stopped the downward decline" in ozone destruction through international controls, he continues, "and we fully expect to see evidence of that in the next 10 years."

But by the time the ozone layer has fully recovered around 2050, he adds, that layer will likely exist in a different climate regime. How it will respond "is a big uncertainty."